Recent Results on 500 MHz Superconducting Cavities at KEK

Two 500 MHz spherical cavities built from Nb were tested. Locations of weak spots at breakdown field levels were detected and surface processing was repeated to remove the weak spots. Q 0 and the accelerating field gradient were improved after each reprocessing. Q 0 =4.1–4.2×10 9 and E acc =6.5 MV/m at 4.2 K, Q 0 =3.2–4.8×10 10 and E acc =7.0 MV/m at 1.8 K were obtained for both cavities.